The use of more than one PTC heater to heat an object is well known to the art and hair curlers have also been developed which utilize resistance heating devices to radiate their heat onto an iron. Generally, hair curlers have utilized multiple nichrome wire heating elements and switchable thermostats to allow for different temperatures of operation. With single temperature units of the prior art, the wattage of the Nichrome heating element is limited in order to provide a steady temperature state in the barrel of the curler. Because the wattage is limited, during the time that the curling iron heats up, the current is limited also and the heat-up time can be rather prolonged. When only one Nichrome wire is used, a current-limiting device is usually connected to it. The current-limiting device adds cost to the hair curler, and is subject to premature failure. When the wattage is increased to increase the rate of heating, a thermostat is needed to limit the steady state temperature. Since the thermostat is a mechanical device, it's life and reliability can be limited and it's use can add cost to the hair curler. When multiple Nicrome wire heating elements are used for dual range temperature settings, the costs are multiplied and the reliability is reduced.
We have found that many of the problems associated with the use of conventional Nichrome wire heating elements can be eliminated by the use of PTC heaters as the heat source. PTC heaters have been in use for many years. Such heaters offer several operating advantages over conventional resistance heating elements in the heating of devices that will impart their heat to other objects. The PTC heater is generally made of a doped barium titinate ceramic which has a sharp positive temperature coefficient of resistance. The PTC heaters are designed such that below a critical temperature, the resistance of the ceramic remains at a low value and is essentially constant or even decrease with increasing temperature. When a predetermined temperature is reached, a crystalline phase change takes plae in the ceramic and this change in crystal structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the heater resistance of several orders of magnitude over a very small temperature range. A barium titinate heater with a room temperature resistance of 3 ohms will increase to 1,000 ohms or more during the crystalline phase change. The temperature at which the crystalline phase change takes place can be adjusted in the PTC heater manufacturing process through the use of appropriate chemical dopents and can be varied between -50.degree. C. and 300.degree. C. When energized with a suitable current by applying voltage to the opposite sides of the PTC heater, the ceramic rapidly heats up to a predetermined operating temperature and then "locks in" at this temperature. This rapid heating is due to the initial low resistance of the PTC heater which results in an internal high power of the heater. The "lock in" is due to the abrupt increase in resistance which causes generated heat to be reduced until it equals dissipated power. At this point, thermal equilibrium is achieved and the heater regulates itself with regard to its temperature characteristics.
The use of two PTC heaters in a heater of a hair curler iron has several advantages. According to the present invention, two PTC heaters with different anomaly temperatures and with welded leads are disposed in the barrel of a hollow aluminum extrusion. The portions of the barrel that are not occupied by the PTC heaters are filled with silicon rubber that is filled with 50 to 80 weight percent magnesium oxide or similar heat transmitting, electrically non-conductive material. The barrel (which may be coated with Teflon, a registered trademark of the DuPont Corporation, to improve its chemical resistance) acts directly as a hair curler and provides for an extremely effective heat transfer from the PTC heaters to the surface of the barrel.
The hair curler heats up extremely rapidly due to the resistance temperature characteristics of the PTC heaters. When the assembly is cold, the PTC heaters are in their low resistance state and cause a high wattage to be developed. This high level of wattage quickly heats the barrel until the resistance of the of the lower anomaly temperature PTC heater begins to increase, thereby decreasing the wattage developed. As the temperature increases and the wattage decreases, an equilibrium condition is reached and the barrel is regulated at the predetermined equilibrium temperature. The cold to hot wattage ratio can be between about 5 to 15 times thereby explaining the significant heat up rate improvement that a PTC heater has over Nichrome wire which is controlled with a wattage limiting device. Lower mass or heat capacity fillers such as magnesium oxide, and the minimal use of the silicon rubber compound reduces the thermal mass of the system and further improves the heating rate.
When two PTC heaters are used in the hair curler, in addition to the extremely fast rate of heating, two different operating temperatures can be readily achieved. When the two PTC heaters are electrically connected in series with a switch set in the low position (FIG. 2a), current must pass through both PTCs and will be limited when the PTC heater with the lower resistance temperature characteristic reaches its anomaly temperature. This limit is not reached until after the entire system begins to heat, and the low temperature-high wattage condition still exists so as not to compromise the rate of heating of the PTC heaters. When the switch is turned to the high position, the low temperature PTC heater no longer limits current to the high temperature PTC heater and the system comes to equilibrium at the temperature associated with the PTC having a higher anomaly temperature.